1. Field of the Invention
This invention relates to microwaveable adhesives. More particularly, this invention relates to hot melt adhesives or heat-curable adhesives that are activated by microwave energy.
2. Background of the Invention
Microwave radiation has become a widely used means for heating objects, particularly food. One advantage of microwave heating is that objects may be heated more quickly relative to conventional heating methods.
Microwave heating involves the portion of the electromagnetic spectrum between infrared and radio waves. Microwaves heat materials having an electric dipole. Water is by far the most common dipolar material. Typically, microwaves pass through an object and tend to align the dipoles. The microwave field reverses itself billions of times a second, which tends to cause the dipoles to rotate. The energy which is released by the rotating dipoles is converted into heat.
Polar materials such as water are not the only microwave receptors. Electrically conductive materials such as metals are also microwave receptors. However, the metal must be very thin or it will reflect almost all of the incident microwave energy rather than absorb the microwave energy.
An electrical conductor of proper thickness undergoes joule heating when irradiated with microwave energy. If a conductive layer is disposed on a nonconductive substrate, the substrate is heated by the transfer of thermal energy from the conductive layer. Joule heating of a conductive layer is generally much more efficient than simple dipole heating and results in heating rates that can be orders of magnitude greater than the heating rates accomplished through the interaction of microwave energy with dipoles.
When microwave energy impinges upon a conductive layer, the microwave energy induces electronic motions that give rise to a current in the conductive layer. Since the conductive layer has a resistance, energy in the form of heat, H, will be dissipated in accord with Joule's law which is EQU H=I.sup.2 R
where I is the current in the conductive layer in amperes and R is the resistance of the layer in ohms. Joule heating, however, takes place only as long as the conductive layer remains electrically continuous. If the conductive layer becomes electrically discontinuous, the current is reduced or eliminated and joule heating is correspondingly reduced or eliminated.
In addition, microwave radiation generated in a microwave oven is not always uniformly distributed throughout the oven. This non-uniformity can give rise to differential heating of the various regions in an object to be heated. Where the amount of microwave radiation is higher, the object heats more rapidly in that region and a hot-spot results.
Microwave radiation can also be used to heat hot-melt and heat-curable adhesives to their melt-flow or activation temperatures. The use of hot-melt adhesives in industry has steadily been increasing in past years, replacing aqueous and solvent-based adhesives. Hot-melt adhesive are particularly preferred because they do not release solvent into the atmosphere and also enjoy rapid set time characteristics. A hot-melt adhesive must be heated to its melt-flow temperature in order to allow the adhesive to flow and bond onto the surface of the adherends. This heating time is generally very short, but may be quite long particularly if the adhesive is in contact with a large mass that can act as a heat sink, or when the adhesive is separated from the heat source by insulating materials. In such circumstances, there exists a need for a method to heat up hot-melt adhesives at a rate faster than conventional thermal heating.
Heat-curable adhesives are adhesives that are chemically activated upon exposure to heat. The adhesives form bonds either when water or solvent is driven off, or when they are cross-linked, crystallized or otherwise initiated after exposure to heat.
U.S. Pat. No. 4,906,497 discloses microwave-activatable hot-melt adhesives. This reference describes an adhesive with an electrically conductive substance blended into the adhesive. The electrically conductive substance heats up faster than the adhesive, transferring the heat to the adhesive.